I've never tried to use this method on anything thicker than 1 .5 times the square of the hole IE approximately 6mm square hole through 10 mm mild steel. and always a long taper a 1/2"20 tpi taper starting tap goes through 1/2" mild steel fairly easy but sometimes starting out with a size smaller tap to initially start the cutting process by removing part of the metal then finishing off with the size you need will reduce the risk of breakage.
This is the reason some of the draw broached I used to have were 30" long, for cutting a .75" dia. 6 spline while broaching through 2" thick materials